The present invention relates to a technical field of a pre-tensioner which is used for a seat belt device installed in a seat of a vehicle such as an automobile and which is actuated to tension a seat belt so as to provide improved occupant restraint in the event of an emergency when a large deceleration is generated, such as during a vehicle collision, and more particularly to a technical field of a pre-tensioner of which a piston is moved with gas pressure of gas generated by a gas generator so as to pull a buckle or a lap anchor, thereby tensioning the seat belt.
Conventionally, vehicles such as automobiles are equipped with seat belt devices. A seat belt device is designed such that, when extremely large deceleration is generated in the event of emergency such as a vehicle collision, an occupant is restrained by a seat belt to inhibit inertia movement of the occupant, thereby protecting the occupant. Some seat belts are equipped with a pre-tensioner which tensions the seat belt in the event of emergency to increase the restraining performance so as to rapidly restrain the occupant with large restraining force. As one of conventional pre-tensioners of a type installed in a seat belt device, a pre-tensioner installed on a buckle side, i.e. a buckle pre-tensioner has been proposed such as, for example, Japanese Patent No. 2507231 (incorporated by reference herein in its entirety).
A pre-tensioner used for a seat belt device disclosed in Japanese Patent No. 2507231 comprises a piston which is slidably and gas-tightly disposed in a cylinder and which is connected to a buckle through a wire, wherein the piston is rapidly moved within the cylinder by gas generated from a gas generator so as to pull the buckle at the event of emergency, thereby tensioning the seat belt.
In the buckle pre-tensioner disclosed in Japanese Patent No. 2507231, an end of the cylinder is a taper portion formed in a truncated cone shape. As the piston rapidly moved by action of gas pressure from the gas generator enters into the end of the cylinder when a safety belt system is not in use, the taper portion of the cylinder is deformed by the piston. Deformation of the taper portion decelerates the piston so as to prevent the piston from running out of the cylinder. That is, the taper portion functions as a piston trapping device.
By the way, in such a pre-tensioner used for a seat belt device, high moving speed of a piston is desired to rapidly tension a seat belt at the event of emergency. At the same time, it is needed to absorb kinetic energy exerted on the piston immediately after operation of the pre-tensioner.
As possible means for absorbing kinetic energy exerted on the piston, there is means using deceleration control of the piston by the structure of the pre-tensioner disclosed in Japanese Patent No. 2507231. However, the deceleration control of the piston disclosed in Japanese Patent No. 2507231 is just a means for preventing the piston from running out of the cylinder and can not effectively absorb the kinetic energy of the piston.